I got a few mails sent to me with the question, why I would be so " negative" about NIRS and lactate.
I am not sure whether critical can be called negative.
Secondly I was and many do not know the story how small point of care lactate analyzer actually moved to north America , but I collected lactate sample s like people collect stamps. (far over 1000'000 sampling )
So not negative at all but somewhat critical.
I like to explain that as short as possible.
a) Lactate is a systemic information gathered somewhere at a certain time. The value you will gather does not tell you any thing more, that there was some where in the body sometimes before you sampled the blood a metabolic situation occurred , where someone part created lactate(. Aerobe or " anaerobe )
We have no clue from the sample how much lactate we created and where , and we have no clue how much lactate from the place where it was created made it to the sampling place and how much was "used" or moved towards another area..
We not even know from a single sample, whether it is a situation, where the next sample will show a higher value or perhaps a lower value or a +- same value..
2. Nirs gives you a local feedback on O2 demand and supply and in some cases when we use a second NIRS at a less involved muscle we can get a systemic trend as well.
. Now this has some limitations as well. You may see a very low SmO2 value close to zero, but the athlete is still moving absolutely easy .
You may see that in a hard workout some muscles may de-saturate , but than as you go harder may reach a flat SmO2 or even may increase SmO2.
So the trend is crucial and not an absolute number. When we combine SmO2 with tHb we have more information why SmO2 did what we see.
. One easy to explain example is in cycling. You may have some athletes they show a very minimal or surprisingly small desaturation trend when using the VL. But they create a lot of wattage ???
Before searching too far it could be , that they use much more hip extension instead of leg extension and therefor show a very minmal SmO2 drop. There can be as well other reasons.
In cycling you may in fact switch muscle groups as you go harder and harder.
So for example your calf muscle may initially work nicely an than will be phased out to a much smaller % of performance,as you go harder.
Or you may switch from leg extension , quadriceps parts to more hip extension like using the gluteal part. this changes will allow to maintain a longer , higher intensity and is easy to be used by perfect coordination in top athletes. Whether they do this as planned or whether it just happens depends on the athlete.
. This switch can create lactate in one part of a muscle but not in another part . So when you compare HHb or O2H or SmO2 and trends like for break points you may find sometimes one and sometimes not at all, but the chances , that lactate and BP of NIRS will have any regular appearance is limited.
. Here a very short summary on what I mean from a great study.
Med Sci Sports Exerc. 2009 Jun;41(6):1277-86. doi: 10.1249/MSS.0b013e31819825f8.
Changes of pedaling technique and muscle coordination during an exhaustive exercise.
Dorel S1, Drouet JM, Couturier A, Champoux Y, Hug F.
- 1Research Mission, Laboratory of Biomechanics and Physiology, National Institute for Sports (INSEP), Paris, France.
Alterations of the mechanical patterns during an exhaustive pedaling exercise have been previously shown. We designed the present study to test the hypothesis that these alterations in the biomechanics of pedaling, which occur during exhaustive exercise, are linked to changes in the activity patterns of lower limb muscles.
Ten well-trained cyclists were tested during a limited time to exhaustion, performing 80% of maximal power tolerated. Pedal force components were measured continuously using instrumented pedals and were synchronized with surface EMG signals measured in 10 lower limb muscles.
The results confirmed most of the alterations of the mechanical patterns previously described in the literature. The magnitude of the root mean squared of the EMG during the complete cycle (RMScycle) for tibialis anterior and gastrocnemius medialis decreased significantly (P < 0.05) from 85% and 75% of Tlim, respectively. A higher RMScycle was obtained for gluteus maximus (P < 0.01) and biceps femoris (P < 0.05) from 75% of Tlim. The k values that resulted from the cross-correlation technique indicated that the activities of six muscles (gastrocnemius medialis, gastrocnemius lateralis, tibialis anterior, vastus lateralis, vastus medialis, and rectus femoris) were shifted forward in the cycle at the end of the exercise.
The large increases in activity for gluteus maximus and biceps femoris, which are in accordance with the increase in force production during the propulsive phase, could be considered as instinctive coordination strategies that compensate for potential fatigue and loss of force of the knee extensors (i.e., vastus lateralis and vastus medialis) by a higher moment of the hip extensors.